WEBVTT 00:00.420 --> 00:04.110 Hello again! In this video, we are going to look at random access to streams. 00:04.650 --> 00:09.720 Normally when we access a stream, we do it sequentially. So we start at the beginning and go through 00:09.720 --> 00:12.300 it step by step until we get to the end. 00:12.690 --> 00:13.380 And then we stop. 00:14.430 --> 00:19.350 But it is possible to access streams randomly, which means we can jump to any position we want. 00:19.650 --> 00:22.230 And read or write data from that position. 00:26.240 --> 00:31.760 The stream classes in C++ have a data member, which keeps track of where the last operation stopped. 00:32.210 --> 00:37.490 That is known as the position marker. If you have performed a write operation, 00:37.550 --> 00:39.140 this will be at the end of the stream. 00:39.140 --> 00:42.080 So just after the data that you've written to the stream. 00:42.890 --> 00:47.000 If you did a read operation, this will be pointing to the following data. 00:47.390 --> 00:52.160 So that is the start of the data that will be retrieved by the next read operation. 00:53.030 --> 00:54.830 And normally that is controlled by the stream. 00:57.670 --> 01:03.370 There are some cases where the programmer can set the position of this marker. If you have a file stream 01:03.370 --> 01:09.460 and it was not opened in append mode, or if you have a string stream. In that case, you can set the position 01:09.460 --> 01:14.950 of the marker. And you can use that to read or write data, anywhere in the stream, because that controls 01:14.950 --> 01:16.960 where the next operation will take place. 01:21.470 --> 01:27.020 The streams have member functions. The seek member functions will change the position of the marker. 01:28.070 --> 01:30.290 They have different names for different streams. 01:30.740 --> 01:35.930 This is because they come from C, which doesn't have the notion of overloaded functions or indeed 01:36.170 --> 01:36.740 functions. 01:37.730 --> 01:41.450 If you have an input stream, then seekg() will set the current position. 01:41.930 --> 01:44.000 So it's 'g' for a "get" operation. 01:44.450 --> 01:50.600 If you have an output stream, then seekp() will set the current position, so that's for a "put" operation. 01:53.790 --> 01:59.340 And there are also member functions which will give you the current position of the marker. tellg() 01:59.340 --> 02:04.590 will give you the position for an input stream and tellp() will give its for an output stream. 02:11.470 --> 02:17.770 These operations are only defined where they make sense, so seekg() and tellg() are only defined for input 02:17.770 --> 02:21.940 streams and seekp() and tellp() are are only defined for output streams. 02:22.840 --> 02:26.170 They are defined for iostreams, but they do not do anything useful. 02:26.770 --> 02:30.130 They give you a runtime error and they put the stream into an invalid state. 02:30.580 --> 02:31.390 So do not do that! 02:33.080 --> 02:40.010 If you have an fstream and it is opened in append mode, then seekp() does nothing. The stream will always write 02:40.040 --> 02:41.690 the output at the end of the file. 02:45.530 --> 02:51.470 The return value from tell is an object of type "pos_type", and this represents a 02:51.470 --> 02:52.450 position in the stream. 02:54.410 --> 03:01.070 It can be converted to an int, so you can print it out or compare it against an int. If the stream is 03:01.070 --> 03:03.500 in an invalid state when you call the tell operation, 03:03.980 --> 03:09.260 this operation will fail, and you can find that by comparing the return value to minus one. 03:10.220 --> 03:15.950 So if you do a tell and the return value is not minus one, then the operation succeeded ,and you have 03:15.950 --> 03:17.330 a valid position marker. 03:22.300 --> 03:27.880 You can pass this result to a seek operation, so you can seek to a particular position. 03:31.080 --> 03:32.520 And that is an absolute position. 03:32.910 --> 03:35.250 You can also seek to a relative position. 03:35.460 --> 03:40.620 So, so many bytes from the beginning or end of the stream, or so many bytes from the current position. 03:41.310 --> 03:48.000 So for example, you can say that's the base position is the end, ios_base::end, and you want to move 03:48.000 --> 03:49.320 minus 10 bytes from that. 03:49.740 --> 03:52.500 So that means 10 bytes before the end of the stream. 03:57.900 --> 04:02.400 One thing you can do is to save the current position of the marker, so you can get it later. 04:02.820 --> 04:09.270 So that is a bit like putting a bookmark in your file or your stream. So you could open some output stream 04:09.270 --> 04:14.400 and send some data to it, and then you save the position of the marker. 04:14.400 --> 04:16.530 So that is where the stream currently is. 04:18.930 --> 04:24.270 And then you can write some more data to the stream. And if you later decide that you want to go back and 04:24.690 --> 04:29.520 undo that output, or overwrite it, then you can seek to the position. 04:29.760 --> 04:33.180 So this is the bookmark and this will return to the bookmark. 04:33.810 --> 04:37.500 And then if you write again, this will overwrite all the data that you put here. 04:38.010 --> 04:39.690 So here is an example. 04:40.470 --> 04:43.140 We open an output, an ostringstream. 04:44.400 --> 04:51.000 We have some data we are going to send to it. And then we call tellp() to get the position marker for 04:51.000 --> 04:51.930 the stringstream. 04:54.580 --> 04:59.260 Then we compare the position of the marker and the number of characters we have written to the stream. 05:00.850 --> 05:04.600 And then with the stream at that position, we then write "hello". 05:05.200 --> 05:09.480 So that is going to make the marker move forward five bytes; five bytes for "hello". 05:10.300 --> 05:12.370 And then we can check the new value of the marker. 05:14.440 --> 05:20.110 Then we print out all the data in the string. And then we go back to the position marker that we saved 05:20.110 --> 05:20.440 earlier. 05:20.860 --> 05:21.970 So that is before 05:21.970 --> 05:22.420 "hello". 05:23.410 --> 05:26.380 And then we put "goodnight" on the stream. 05:26.800 --> 05:29.170 So this is going to overwrite "hello" with "goodnight". 05:29.920 --> 05:30.940 So let's see what happens. 05:36.450 --> 05:41.970 So we wrote 18 characters to the stream, so "It is time to say" has 18 characters. 05:42.630 --> 05:49.200 The stream marker is 18 bytes into the stream. So 'I' goes at byte 0, 't' goes at byte one and so on, and the 05:49.200 --> 05:51.260 last character went at byte 17. 05:51.900 --> 05:54.750 So the next operation will be at byte number 18. 05:55.920 --> 05:59.900 Then we send another five bytes to the stream, so the marker moves ahead five bytes. 05:59.910 --> 06:05.160 It is now at position number 23, and we have a string which says "It is time to say hello". 06:06.980 --> 06:13.340 Then we go back to the position before we wrote "hello", and we write, "goodnight", so we have 06:13.340 --> 06:13.730 overwritten 06:13.730 --> 06:14.900 "hello" with "goodnight". 06:17.230 --> 06:21.010 So you could use this with a file stream to modify a file in place. 06:22.450 --> 06:25.900 However, the best way to modify a file is actually to read it into memory. 06:26.170 --> 06:31.450 Make your changes in memory and then write them over the original file. So you could, for example, 06:31.510 --> 06:33.580 read the data into an istringstream. 06:34.390 --> 06:40.720 Then you get the bound string, and make the changes to the data, in the string, and then you write that 06:40.720 --> 06:42.880 string over the original file. 06:46.300 --> 06:49.390 If you use seek and tell, this will modify the file in place. 06:50.200 --> 06:54.800 It is more complicated, it is harder to get right and if you do make a mistake, then you have overwritten 06:54.820 --> 06:58.900 the data, so the original file is no longer available. Unless you made a back up. 07:00.430 --> 07:04.840 This could be useful, though, if you have a very large file, or a very limited amount of memory, and 07:04.840 --> 07:08.200 you cannot read the entire contents of the file into memory in one go. 07:10.970 --> 07:15.290 Okay, so that is it for this video, it is time for me to say goodnight. Or rather, good afternoon! 07:15.920 --> 07:16.790 I will see you next time. 07:17.090 --> 07:18.770 Meanwhile, keep coding!